The screen printing process or screen process, also called silk screen process or serigraphy is a process of printing through the unblocked areas, called screen cells, of a metal or fibre screen, with a free flowing ink (screen printing ink) which is spread and forced throughout the screen by means of a squeegee. This technique is frequently used in the production of coloured posters, show cards, decalcomania, printed circuits etc. Originally the screen was made almost exclusively of silk. Natural silk meshes, however, are woven from stranded threads and have irregularities and a rough surface structure. Most widely used materials today are nylon, terylene and metal. Serigraphy is literally drawing on silk. By screen printing various kinds of support materials or carriers can be printed, in most cases using ultraviolet light drying screen printing inks. Often claimed benefits are colour stability for fading, and insensitivity for scratches. In general, these are prints fit for use in harsh environmental or manipulation conditions. Rotational or rotary screen production is 5 to 10 times faster than flat screen printing but is handicapped by resolution limits. Common screen printing productivity for quality work is low: flat screen printing has a typical production rate of 4 m/min, rotational screen printing has a rate of 20 to 40 m/min. This may be compared to flexo printing at a typical printing rate of 150 m/min, or a maximum printing rate of 300 m/min.
Screens for use in the screen printing process comprise a plurality of screen cells, through which the screen printing ink may flow. These screen cells may be arranged either orthogonally or hexagonally. According to an orthogonal arrangement of the screen cells, also referred to as a 90.degree. geometry, screen cells have a rectangular or square shape and are arranged side by side adjacent to each other in a regular rectangular or square grid. Screen cells may be formed by two orthogonal sets of parallel wires, typically for silk screens. According to a hexagonal arrangement, screen cells have the shape of an equilateral hexagon and are arranged side by side in three directions: horizontally, at 60.degree. and at 120.degree.. This arrangement is also referred to as a 60.degree. geometry, symmetry or screen and may be realised by a metal screen. The screen pitch, i.e. the shortest distance between the centres of two adjacent screen cells, depends on the technology and the geometry used. A typical pitch value for currently used metal screens is 83 .mu.m, whereas silk screens may have a 55 .mu.m period, which corresponds to 180 wires/cm.
In screen printing, colour images may be reproduced by decomposing or separating the original colour image in a number of screen print colour components. These separated colour components are in general printed in juxtaposition (side by side) or in solid overprints, using the same type of screens for each colour component. Each screen will subsequently print part of the colour image with the appropriate print colour. Halftone techniques may be used to create colour shades. It is emphasized that in the rest of the text "screening", "screen" etc. refers to the screen printing process, whereas "halftoning", "halftone" (in other literature also referred to as "screening") refers to the process where varying densities are obtained by varying the spatial distribution of halftone dots, the halftone dots being realised by a binary process: i.e. ink or no ink, an open or closed screen cell in the screen material, softened or hardened clusters on a chemical or photosensitive substrate etc. Colour shades may be varying between the colour of the base or carrier, e.g. paper or textile, and the solid ink colour. With the more traditional halftoning techniques, halftone dots are arranged in a uniform grid of halftone cells and the different shades are obtained by varying the size of the halftone dot per unit area of the halftone cell.
By application of a halftone image to a screen, the halftone dots, making up the halftone image, will be re-sized by the screen structure, depending on the screen pitch versus the size of halftone dot used, and only an integer number of screen cells will be covered. This will reduce (quantization effect) the number of shades that can be reproduced. The halftone pattern will further interfere with the screen pattern due to the periodic structure of both the screen and the halftoning method, in turn the halftoned colour overprints will interfere because the same screen structure is used in printing, both resulting in moire patterns in the print. Colour stability will suffer from minute register variations between the subsequent colour prints due to varying overlap of the coloured dots. This situation puts practical limits for the halftone-screen printing combinations to guarantee fluent tone transitions and to prevent objectionable moire patterns or colour inaccuracies in the print. In order to avoid the above mentioned problems, for coarse screens, the use of halftoned overprints will be avoided.